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Study of feed horn solutions for IrbeneRT-32 radio telescope
Marcis Bleiders
Ventspils International Radio Astronomy Center of Ventspils University College
BAASP 2019 – 6th International Scientific Conference
Ventspils, Latvia
Contents of presentation
• Main requirements for RT-32 geometry
• Employed analysis and optimization techniques
• Investigation of varous feed horns –profiles and far field performance
• Performance comparison and conclusions
2
Secondary focus feed horn for RT-32
• Main requirements:
Secondary mirror subtended angle 21°
Equivalent f/D ratio 2.7
Aplitude taper at secondary mirror edge 13 dB
Feed apperture phase error ≈1.3 rad (0.1…0.3 𝜆)
Main lobe full bandwidth 10.6° @ -3dB, 21° @ 10…13 dB
Directivity 25 dBi
Maximum cross-polarization level <-20 dB (𝑒𝑓𝑓𝑝𝑜𝑙>0.99)
Sidelobe level & co-polar beam <-25 dB with good E/H plane symmetry
Input return loss <-25 dB
Bandwidth >50 % (octave is 67 % fractional BW)
Other Easy to manufacture, compact size
3
Initial estimation of feed dimensions
• Estimation is carried using Gaussian beam technique [P. F. Goldsmith, QuasiopticalSystems, 1998]
• Radius of beam waist:
𝑊0 = 0.216 |𝑇(𝑑𝐵)|𝑓
𝐷, where 𝑇 is edge taper
• Aperture radius and slant length, which maximizes coupling to fundamental Gaussian beam mode:
𝑎𝑎𝑝𝑝 = 𝑊01+0.172𝛽2
0.644, 𝐿 =
𝜋𝑎𝑎𝑝𝑝2
𝜆𝛽, where 𝛽 is aperture phase error
• Using previously shown parameters for RT-32 (𝑓
𝐷= 2.7, T = 13 dB, 𝛽 = 1.3 rad) :
𝒂𝒂𝒑𝒑 ≈ 𝟑. 𝟑𝝀 , L ≈ 𝟐𝟐…𝟐𝟓𝝀
• Optimum radius of input waveguide (above TE11 and below TM11 cutoff):
𝒂𝒊𝒏 = 𝟎. 𝟒𝟖𝝀4
𝝀 = 𝟏𝟖 𝒄𝒎
𝝀 = 𝟑. 𝟓 𝒄𝒎
[Wylde et al. 1984]
Beam at RT-32 secondary focus
Ideal Gaussian beam pattern for RT-32
5
Edge taper: 13 dB
Current RT-32 C-X band horn
6
• Length: 102 cm (23 𝜆 @ 6.7 GHz )• Radius of aperture: 24 cm (5.3 𝜆 @ 6.7 GHz )
4.5 GHz, 𝜑 = 0˚
4.5 GHz, 𝜑 = 90˚ 6.7 GHz, 𝜑 = 90˚
6.7 GHz, 𝜑 = 0˚ 8.8 GHz, 𝜑 = 0˚
8.8 GHz, 𝜑 = 90˚
Actual measurements
Investigated feed horns
1. Classical dual-mode conical Potter horn
2. Smooth discrete section shaped horn
3. Classical conical corrugated horn
4. Compact profiled corrugated horn
7
Analysis method – Mode matching
[L]
[S]𝒃𝒊𝑳
𝒃𝒋𝑺
𝒂𝒊𝑳
𝒂𝒋𝑺
ො𝒛
𝐸𝑡𝐿,𝑆 =
𝑖=1
𝑁𝐿,𝑆
(𝑎𝑖𝐿,𝑆 + 𝑏𝑖
𝐿,𝑆)𝒆𝒊𝑳,𝑺
𝐻𝑡𝐿,𝑆 =
𝑖=1
𝑁𝐿,𝑆
(𝑎𝑖𝐿,𝑆 − 𝑏𝑖
𝐿,𝑆)𝒉𝒊𝑳,𝑺
𝑩𝑳
𝑩𝑺= 𝑺
𝑨𝑳𝑨𝑺
𝑒−𝛾𝑛𝑙𝑺𝒏 𝑺𝒏+𝟏
𝑬(𝐑, 𝜽, 𝝋) =𝒋𝒌𝒆−𝒋𝒌𝑹
4𝝅𝑹(1 + 𝐜𝐨𝐬 𝜽 )𝒂2න
0
1
න0
2𝝅
𝑬𝒂(𝒓, 𝝋′)𝒆𝒋𝒌𝒂𝒓𝒔𝒊𝒏 𝜽 𝐜𝐨 𝐬 𝝋−𝝋′
𝒓𝒅𝒓𝒅𝝋′
𝑬𝒂(𝒓,𝝋′)
𝑬(𝐑, 𝜽, 𝝋)
𝑇𝐸11 𝑧
𝑟
Field matching on waveguide interface and scattering matrix calculation: Complex device approximation by cascading multiple waveguide steps:
Far field calculation from obtained aperture field distribution:Comparison of MATLAB implementation against CST MWS:
8
Optimization method – Genetic Algorithm (GA)
• GA searches for parameters which gives minimum of fitness (objective)function.
• Already available in MATLAB
• In this work, optimization for best cross polarization is carried out.
9
Genetic Algorithm
Horn Profile generation
Mode Matching
Far field calculation
Y = Goal -Result
Profile parameters Y
Fitness function
Antenna #1 – Potter dual-mode horn
+ =
TE11 (P≈ 𝟖𝟒%) TM11 (P≈ 𝟏𝟔%)
10
3.3𝜆
22𝜆
0.64𝜆0.48𝜆 0.56𝜆
2 x 0.045𝜆
[P.D. Potter, A new horn antenna with suppressed sidelobes and equal beamwidths, 1976]
Antenna #1 Far field patterns
Antenna #2 – Smooth discrete section shaped horn
12
• Optimized for best average cross-polarization at multiple normalized frequencies. Fitness function:𝑌 = 𝐺𝑋𝑝𝑜𝑙 − 𝐴𝑉𝐺([𝑓𝑠𝑡𝑎𝑟𝑡 …𝑓𝑠𝑡𝑜𝑝])
Resulting profile
22𝜆
0.48𝜆
3.3𝜆
11.64𝜆1.75𝜆
6.61𝜆1.54𝜆
3.75𝜆
13
Antenna #2 Far field patterns
Antenna #3 – conical corrugated horn
3.52𝜆 18.48𝜆
3.3𝜆
0.47𝜆
≈ 0.25𝜆
3 corrugations per 𝝀
• Optimized for best cross-polarization at center frequency
0.48𝜆
14
R
Antenna #3 Far field patterns
15
16
Antenna #4 – Compact profiled corrugated horn• Optimized for best average cross-polarization with reduced spread at multiple normalized frequencies.
Beamwidth optimization added. Fitness function:𝑌 = 𝐺𝑋𝑝𝑜𝑙 − 𝐴𝑉𝐺𝑋𝑝𝑜𝑙 ([𝑓𝑠𝑡𝑎𝑟𝑡 …𝑓𝑠𝑡𝑜𝑝]) + 𝑆𝑇𝐷𝑋𝑝𝑜𝑙([𝑓𝑠𝑡𝑎𝑟𝑡 …𝑓𝑠𝑡𝑜𝑝]) + 3 ∗ |𝐺𝐵𝑊 − 𝐴𝑉𝐺𝐵𝑊([𝑓𝑠𝑡𝑎𝑟𝑡 …𝑓𝑠𝑡𝑜𝑝])|
Resulting profile
2.84𝜆 2.91𝜆 7.72𝜆 1.53𝜆
3.3𝜆3.24𝜆
1.67𝜆0.8𝜆
4 corrugations per 𝝀
T𝐨𝐭𝐚𝐥 𝐥𝐞𝐧𝐠𝐭𝐡: 𝐨𝐧𝐥𝐲 𝟏𝟓𝝀
17
Antenna #4 Far field patterns
Antenna #4 MM vs. CST MWS
18
CST Time Domain
CST Frequency Domain
Profile summary
19
#1 #2 #3 #4
Performance summary
20
Phase center position
21
• Following phase center definition is used – point in horn which gives minimum phase deviation withinilluminated reflector (secondary mirror in this case) subtended angle. This point should be positioned in focus.
Feed efficiencies
22
Antenna #2
Antenna #3 Antenna #4
Antenna #1
Conclusions1. Dual-mode conical Potter horn
Advantages: Simplest to make and lightweight. Suitable for very low or very high frequencies
Disadvantages: Very narrowband.
2. Smooth discrete section shaped horn:
Advantages: Improved bandwidth, and overall performance. Lightweight. Length potentially could be reduced with more optimization and different profile options. Good candidate for RT-32 L/S band feed horn
Disadvantages: Must maintain sharp and accurate flare steps and angles.
3. Conical corrugated horn:
Advantages: Great performance, cross polarization >25 dB, low sidelobes and equal E/H plane beam widths over almost octave bandwidth.
Disadvantages: Expensive to make, Heavy.
4. Compact discrete profiled corrugated horn
Advantages: 30 % smaller length, but similar performance to corrugated horn antenna #3.
Disadvantages: High fabrication accuracy must be maintained
23
Thank You for the attention! Any questions?
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